Indoor and outdoor air quality has become an important occupational health, political, scientific, and environmental topic over the past two decades. Air and gas streams carry various particles therein. Removal of these particles improves air quality and reduces the risk of infection or other ailments that result from air pollution caused by these airborne particles. Good air quality is particularly important for those who suffer from respiratory ailments such as asthma.
One of the major sources of infection is from airborne microbes when soil, water, dust, and decaying organic matter are disturbed. They can be carried indoors by any number of vehicles, including people, air currents, water, equipment, or construction materials. Once indoors, the attendant microorganisms can proliferate in a variety of indoor ecological niches and, if subsequently disbursed into the air, can serve as a source for airborne infections.
Additionally, pathogens such as the influenza virus, rhinoviruses, adenoviruses, respiratory syncytial virus (RSV), tuberculosis, and the measles virus can be spread by aerosolized oral and nasal secretions. Often, these pathogens are contained in either droplets or droplet nuclei ranging in size from 1 μm-5 μm. These droplets remain suspended indefinitely in air and can be transported over long distances. The dispersal of virus into air can be exacerbated by coughing and sneezing that spreads a cloud of pathogens into the air.
Furthermore, many industries generate significant amounts of liquid aerosols that contribute to indoor air pollution. Examples of these liquid aerosols are: metal working fluid mists generated by mechanical industries; paint mists generated in the automobile industry; pesticides used in the agricultural industry; ink mists generated in the graphical industry; and acid mists generated in the chemical industry. Not only do these liquid aerosols have adverse affects on workers' health, but environmental standards are increasingly more stringent and require more efficient filters for reducing air pollution and improving air quality.
Airborne transmission of particulate matter, including those in liquid aerosols, is also especially problematic in healthcare facilities, contributing to the approximately 103,000 deaths annually caused by infection in U.S. hospitals. Susceptibility to these airborne pathogens is greatest among immune deficient patients such as the elderly, burn patients, and those receiving implants or chemotherapy treatments. Surgeons and other healthcare professionals are also exposed to pathogens carried by liquid aerosols into the operating room, for example, and run the risk of these pathogens reaching the nasal mucosa. For example, the air can be polluted by viruses and bacteria, including human papilloma virus (HPV), HIV, and Staphylococcus, that are released in the laser plume of surgical equipment used to section bone. In another example, legionella spp. are commonly found in warm water distribution systems and can be disseminated into the air space above it. Locally produced distilled water provides an environment in which legionella can multiply. In several hospital outbreaks, healthcare providers have determined that patients were infected by exposure to contaminated aerosols generated by cooling towers, showers, faucets, respiratory therapy equipment, and room air humidifiers.
Air filters are one tool that consumers, industry, and healthcare facilities alike rely on to improve air quality. For example, many consumers use home air purifiers or filters in their vacuum cleaners to improve air quality in the home. Healthcare providers and those working in industry often rely on face masks to protect them from airborne particulate matter and pathogens. A commonly used type of air and gas filter is one that has a HEPA (High Efficiency Particle Air) filtration media. HEPA filtration media are capable of retaining >99.97% of 0.3 μm particles and consist of a non-woven sheet composed of glass and/or polymeric fibers ranging in diameter from about 0.5 to about 10 μm. These filters are used primarily in collective protection (room) filter systems, although they may also be used in respirators. Ultra Low Penetration Air (ULPA) filtration media are capable of retaining 99.99% of a specified particle size at a specified media velocity. SULPA (Super ULPA) filters are available for use in environments where maximum cleanliness is required. These filters have an efficiency of 99.9999% on the same basis as ULPA filters.
Despite HEPA air filter's exceptional retention rate for particulate matter in air or gas streams, these conventional HEPA filter media are susceptible to penetration by liquids, thereby limiting their effectiveness for capturing or retaining pathogens from liquid aerosols. Liquid aerosol clogging occurs when liquid particles, particularly water aerosols, collect on the fibers and create a thin film that covers each fiber. When this film joins two or more fibers together, pools and bridges form that restrict flow and rapidly increase the pressure drop, thereby causing a resultant decrease in filter efficiency. As such, those relying on filters to protect them from pathogens and other particles present in liquid aerosols remain susceptible to infection because currently available filtration media function less efficiently under such circumstances.
The use of nano fibers distributed over microglass fibers for filtering sub-micron particles from water is known in the art. However, such filters have high pressure drops that negate their effectiveness as an air or gas filter. For example U.S. Pat. No. 6,838,005 describes a nano alumina filter that is effective for filtering virus from water. Until the present invention, it was generally believed that any attempts to lower the pressure drop in nano alumina filters would require pore sizes that were far too large to effectively filter fine particles from the air. Further, it was presumed that bulk water was necessary to effect the nano alumina's zeta potential and therefore its electrostatic benefits, thereby negating its use as an air filter.
Additionally, the energy consumed in overcoming the pressure drop in a filter is often more than the cost of the filter itself. With a HEPA filter system, the energy consumed can be four to five times the initial cost of the filter. Therefore, a filter that reduces the pressure drop over the whole life cycle of the filter would provide significant savings. Furthermore, in instances where the filter material is used in a medical application or where it might contain bacteria, then the waste disposal costs escalate rapidly because the filter material is considered to be biohazardous waste. As such, a longer life filter minimizes the frequency of disposing of biohazardous filters and therefore reduces costs.
Given this, there is a need among consumers, healthcare facilities, and other industries for a cost-effective high efficiency filter that retains particles at a level that is at least as high as HEPA filters, but that is also able to intercept water-aerosolized bacteria and that would be superior to conventional HEPA filters for air purification. Such filters would be particularly beneficial for air purification in environments such as hospitals and health care facilities, in pharmaceutical settings such as during drug preparation, in biological safety hoods, and for generally removing mold, fungus and mildew spores from the air and liquid aerosols. Such filters would also be beneficial in collective protection and in personal respirators, such as for protecting military personnel from biological attack, for protecting the homeland from a terrorist attack that utilizes bacteria or viruses, and/or during clean-up of attacked sites such as the World Trade Center.